Soil compaction measurement

The in-situ measurement of the shear modulus of a surface layer of soil (12) is performed by apparatus in which a contact foot (11) with a preselected downward bias force engages a defined area of a region of soil. A drive transducer (13) shakes the contact foot (11) vertically and sensing transducers (17, 18) determine the force applied to the foot (11) and the resulting movement of the foot (11) and generate respective first and second output signals. The in-phase movement responsive output signal is analyzed in relation to the force signal to obtain a measurement of a single parameter which is representative of the stiffness provided by the soil (12). The shear modulus as well as the void ratio of the soil can be calculated from the stiffness. The drive transducer (13) is energized by an a.c. signal which progressively varies in frequency over a preselected band with the rate of change also varying with frequency thereby providing a greater force spectrum content at lower frequencies than at higher frequencies in the band, while maintaining a substantially constant force magnitude.